Silicon wafers used in making integrated circuits begin as single crystal four, five or eight inch discs worth $50-200 and after final processing may be worth upwards of $18,000. It is therefore essential that close track be kept of them through as many as four hundred different process steps. To track these wafers they are marked and may be read by OCR readers which cost about $50,000 each or by human eye, neither of which is highly reliable. It has been suggested to use bar codes and bar code readers, but this approach too has a number of shortcomings.
The bars are formed on the wafer by etching with a laser beam, for example, and the spaces are the unetched areas between the bars which are the wafer surface. In general the spaces reflect scanner light highly specularly while the etched bars reflect diffusely but both are extremely strong, i.e., bright signals, so that there is little contrast between them. It has been recognized, however, in U.S. patent application Ser. No. 450,243, Improved Bar Code Reader System, filed Dec. 13, 1989, in the name of the same inventor, that since laser light is polarized and since specularly reflected light maintains the original laser beam polarization, a crossed-polarizer could be used to block that specularly reflected light. This virtually completely "throws away" the space signals by making them appear black but the diffuse signal from the bars loses only a small portion of its energy to the crossed-polarizer while the remainder is detected and is in strong contrast to the specular signals. Thus the code signals have been essentially reversed so that black appears white and white, black. However, there have appeared still further problems.
Various coatings, processings and imperfections such as underheating and puddling can cause the bars or portions thereof to reflect specularly as a space, confusing the system into recognizing a wider space when in fact it is scanning a bar.
Imperfections in the bar code surface can make local areas of a bar and of a space appear as the other and result in false signals. For example, a bar with a "space-like" imperfection in the middle may be read as a bar, a space and another bar--three signals where there should be but one.
During the processing, through as many as 400 different process steps, the wafer surface undergoes various coatings and finishes. Each of these can vary the reflectivity characterization of the surface so that the signal ratio from state to state of a bar or space may vary by a factor of 1000:1.
Finally, if the laser beam dot is not controlled properly the dots will not be made to overlap and form a proper bar: there will be unetched spaces between the etched dots along the bar. A scanner traversing the bar in the unetched portions of the bar will respond as if it has read a space.